<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>

  <style>
    .app {
      height: 300px;
      width: 280px;

      display: flex;
      flex-wrap: wrap;
      background-color: #bfa;
    }

    .box {
      width: 100px;
      height: 100px;
      background-color: tomato;
      border: 2px solid blue;
    }
  </style>

</head>

<body>

  <div class="app">
    <div class="box">1</div>
    <div class="box">2</div>
    <div class="box">3</div>
  </div>
</body>

</html>